Many conventional differential gears employ a casing-shaped ring gear that accommodates and supports input and output bevel gears. As a result, the whole assembly is inevitably large-sized and heavy-weighed. To make the assembly compact, light weight and less costly, Japanese Unexamined Utility Model Publication No. 169642/1989 and U.S. Pat. No. 4,232,569, for example, have proposed differential gears having a simple disc-shaped ring gear with bevel gears being exposed from the ring gear.
However, these differential gears could not provide a limited slip effect. For example, when one ground wheel is stuck in muddy ground, most of the rotational force from the ring gear is wasted only to cause the stuck wheel to spin because little external load is applied on the stuck wheel. So, it was very difficult for a vehicle to escape from the muddy ground.
Japanese Unexamined Utility Model Publication Nos. 17541/1992 and 34248/1975 and U.S. Pat. No. 2,354,214, for example, have proposed differential gears that can provide a limited-slippage effect.
However, all of these differential gears employ a casing-shaped ring gear so that rotational resistance applied on the heavier load bearing wheel and its shaft is transmitted to the lighter load bearing wheel and its shaft via the ring gear to prevent spinning of the lighter load bearing wheel. The whole assembly is inevitably large-sized and complex, which results in an increased production cost.